Anomalous electrochemical capacitance in Mott-insulator titanium sesquioxide

Electrochemical capacitors with pure electric double-layer capacitance have so far been largely limited to carbon materials. Conventional metal oxides with faradaic pseudocapacitance substantially suffer from material instability at high temperatures, and thus there is a demand for novel metal oxides exhibiting improved high electrochemical capacitance with material stability. In this study, titanium sesquioxide, Ti₂O₃, a Mott-insulator in its metallic state at 300 °C, shows an anomalous increase of ∼560% in its solid-state electrochemical capacitance as compared to its pristine response at room temperature (RT). In an aqueous electrolyte, a maximum capacitance of 1053 μF cm⁻² was obtained, which is 307% higher than that with solid-state electrolytes. Moreover, the semiconducting state of Ti₂O₃ demonstrated ∼3500% enhancement in its electrochemical capacitance upon infrared illumination at RT. The observed electrochemical responses of Ti₂O₃ are attributed to the transition from the semiconducting state to the metallic state by redistribution of localized electrons. Our experimental results find a good agreement with the first-principles density functional theory calculations that revealed an increase in charge density with a rise in temperature, which is mainly attributed to the surface Ti atoms. The study opens up wide avenues to engineer the electrochemical capacitance of Ti₂O₃ with high thermal stability.